In NASA's subsequent explorations of Mars, the agency has stuck to one directive: Follow the water. The Mars Phoenix Lander directly sampled water ice last year, and orbiters and rovers have found ample evidence that liquid water once flowed on the planet.

Researchers believe that the Jezero Crater housed a lake slightly larger than Lake Tahoe during Mars's Noachian epoch, between 3.8 and 4.6 billion years ago. Long-gone Martian rivers carried claylike materials, shown in green in this color-enhanced image, into the lake to form a large delta.